The formation of inorganic soot, particularly metallic oxide soot, produced by reacting a precursor in the flame of a burner is well known. For example, soot generated by such a reaction has been used to form articles such as crucibles, tubing, lenses, and optical waveguides by depositing the soot on a receptor surface.
This process is particularly useful for the formation of optical waveguide preforms made from doped and undoped silica soot, including planar waveguides and waveguide fibers. The waveguide formation process generally involves delivering a silicon-containing precursor to a burner and reacting the precursor in a burner flame generated by a combustible gas such as a mixture of methane and oxygen. Historically, halide-containing precursors, such as silicon tetrachloride and mixtures of silicon tetrachloride with various dopants have been used for producing waveguide preforms by vapor phase deposition techniques such as, for example, VAD (vapor axial deposition) and OVD (outside vapor deposition).
In these procedures, typically a vapor delivery process is utilized in which halide-containing raw materials are vaporized at a location remote from the burner. The vaporized raw materials are then transported to the burner by a carrier gas where they are volatilized and hydrolyzed to produce soot particles which are collected on a receptor surface. The receptor surface may be a flat substrate in the case of planar waveguide fabrication, a rotating starting rod (bait tube) in the case of VAD for waveguide fiber fabrication, or a rotating mandrel in the case of OVD for waveguide fiber fabrication. In some OVD systems, the cladding portion of the waveguide preform is deposited on a previously formed core preform, instead of on a mandrel.
Because of the deleterious environmental effects associated with the use of halide-containing precursors, as described in Cain et al., U.S. Pat. No. 5,599,371 and Dobbins et al., U.S. Pat. No. 5,043,002, halide-free silicon-containing raw materials have been proposed as alternative precursors for forming waveguide preforms. In particular, as described in the Dobbins et al. patent, the relevant portions of which are incorporated by reference, polymethylsiloxanes are preferred precursor materials, with polymethylcyclosiloxanes being particularly preferred, and octamethylcyclotetrasiloxane being especially preferred. Blackwell et al., U.S. Pat. No. 5,152,819, which is incorporated herein by reference, describes additional halide-free silicon compounds, in particular, organosilicon-nitrogen compounds having a basic Si—N—Si structure, siloxasilazanes having a basic Si—N—Si—O—Si structure, and mixtures thereof, which may be used to produce high purity fused silica glass. Both the Dobbins and Blackwell patents disclose vaporizing the halide-free precursor at a location remote from the burner, transporting the vaporized precursor to the burner using a carrier gas, and combusting the vaporized precursor in a burner.
Hawtof et al. U.S. patent application Ser. No. 08/767,653, filed on Dec. 17, 1996 and entitled “Method and Apparatus for Forming Fused Silica by Combustion of Liquid Reactants,” the contents of which are incorporated by reference, discloses that delivery of a vaporized polyalkylsiloxane feedstock to a burner can be problematic. Specifically, high molecular weight species can be deposited as gel in the line carrying the vaporous precursors to the burner or within the burner itself. This gelling reduces the soot deposition rate, and during optical waveguide preform manufacture, leads to imperfections in the preform that will produce defective or unusable optical waveguide. Hawtof et al. U.S. application Ser. No. 08/767,653 discloses that this gelling of polyalkylsiloxane can be overcome by delivering the polyalkylsiloxane in liquid form to the burner and atomizing the liquid precursor at or proximate to the burner.
Numerous burner designs have been developed for use in vapor delivery processes, examples of which can be found in Moltzan et al., U.S. Pat. No. 3,642,521, Powers, U.S. Pat. No. 4,165,223, Moltzan U.S. Pat. No. 3,565,345, Moltzan U.S. Pat. No. 3,698,936, and Cain et al., U.S. Pat. No. 5,599,371. The previously discussed Hawtof et al. U.S. Patent application discloses a burner design for use in a liquid delivery precursor process.
Whether the precursor is delivered to the burner in vapor form or liquid form, it is important that the burner provides a distributed, even stream of precursor to be reacted in the flame of the burner to form the soot which is deposited on the receptor surface. This consideration is particularly important during waveguide manufacture to form accurate refractive index profiles. Current burners are typically manufactured using metal machining technology. One disadvantage of current burner design is that it is very difficult to manufacture burners for deposition of metal oxide soot having orifices and supply channels on a miniaturized scale, i.e. channels and orifices having widths or diameters less than 150 microns. Variability of mass produced parts is also problem with current burner technology, in addition to the cost in machining precision channels and orifices on a miniaturized scale.
In addition to the above general considerations for burners, for liquid delivery burners in which the liquid precursor is atomized, uniform orifice size and accurate dimensions of the burner channels and orifices are key features in achieving targeted and uniform droplet size which is extremely important in complete combustion of the precursor materials. Burners used in liquid delivery systems are typically separately fabricated and later integrated with the atomizing device which atomizes the liquid prior to combustion. Conventional atomizing devices typically provide a spray having a large droplet size distribution. The larger droplets are difficult to combust, and uncombusted droplets can cause defects in the product made by the deposited soot.
In view of these disadvantages, it would be desirable provide a burner produced by micromachining procedures to provide micron size orifices, channels and tolerances that could be consistently reproduced. It would also be useful to provide a burner for use in a liquid delivery system which includes a liquid atomizer formed as part of the burner, with the atomizer producing small droplets and a narrow droplet size distribution to enable efficient combustion of liquid precursor droplets.